Overheads
(1) Sound energy is radiated uniformly in all directions from a small source at a rate of 1.2 w. (a)
What is the intensity of the sound at a point 25 m from the source? (b) What is the relative
intensity (the decibel measure) at this reception point?
Assuming no absorption of sound energy by the transmitting medium, the 1.2 w of sound power
is associated with a spherical area having a radius of 25 m.
P
P
1.2w
w
I=A =
=
= 1.5x10-8 2
2
2
4r
4(2500cm)
cm
I
1.5 x 10-8 w/cm2
 = 10 log I = 10 log
10-16 w/cm2
o
= 10 log (1.5 x 108)
 = 10 (log 1.5 + log 108) = 10 (0.1761 + 8) = 82 db
type of sound
threshold of hearing
whisper
very soft music
average residence
conversation
heavy street traffic
thunder
threshold of pain
jet engine
intensity (db)
0
10-20
30
40-50
60-70
70-80
110
120
170
4.5.1 Overheads
(1) Sound energy is radiated uniformly in all directions from a small source at a rate of 1.2 w. (a)
What is the intensity of the sound at a point 25 m from the source? (b) What is the relative
intensity (the decibel measure) at this reception point?
What general principles should be followed when solving problems using the overhead projector
or black/white board?
(1)
(2)
(3)
(4)
(5)
(6)
4.5.2 Overheads
4.5.3 Overheads